Modified chromium-containing catalyst composition for polymerizing alpha-olefins and method of preparing same

ABSTRACT

A modified supported catalyst for polymerizing alpha-olefins, for example, ethylene, is prepared by combining a chromium compound-containing catalyst with a modifier which is an oxide of an element of Group II A of the Periodic Chart of the Elements. The resulting modified catalyst composition, when used to polymerize alpha-olefins, produces polymer resins having higher values of High Load Melt Index (I 21 ) than catalyst compositions without the modifier. 
     There is also disclosed a process for polymerizing alpha-olefins, at relatively lower temperatures than was heretofore possible, to obtain resins of relatively high HLMI, using the above-identified modified catalyst composition. The resulting polymers can be used for the fabrication of blow-molded products, e.g., household and industrial containers.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention is directed to a chromium compound-containingcatalyst for polymerizing alpha-olefins, also known as 1-olefins, to amethod for producing such a catalyst and to a method of polymerizingsuch olefins. More particularly, the invention is directed to a methodof producing resins which are polymers of ethylene or copolymers ofethylene and at least one C₃ -C₁₀ alpha-olefin, which have relativelyhigh values of high load melt index (HLMI).

2. Description of the Prior Art

Chromium compound-containing alpha-olefin polymerization catalysts, alsoknown as Phillips catalysts, have been extensively described in theliterature. They are formed by supporting chromium trioxide, or acompound calcinable thereto, on a refractory oxide support material, forexample, silica, alumina, zirconia, thoria or silica-alumina, andheating the oxide support material in a non-reducing atmosphere,preferably an oxidizing atmosphere, to produce an active polymerizationcatalyst. The produced catalyst is used to polymerize 1-olefins usingthe so-called "solution form" or "particle form" process. In the"solution form" process, the monomeric 1-olefin, which is normallyethylene or a mixture of ethylene with up to about 40 wt. % of other1-olefins, is contacted with a suspension of the catalyst in a liquidhydrocarbon which is a solvent for the polymer at the polymerizationtemperature employed. In the "particle form" process, the monomer1-olefin is contacted with a suspension or a fluidized bed of thecatalyst particles in a fluid medium under conditions such that thepolymeric 1-olefin forms as solid particles suspended in or fluidized inthe fluid medium. The fluid medium can be, for example, a liquidhydrocarbon or a gas. Examples of suitable liquid hydrocarbons areisobutane and n-pentane. Examples of suitable gases are nitrogen orargon mixed with the gaseous monomer, or the undiluted gaseous monomer.Processes of this type are described in, for example, U.K. publishedpatent specification Nos. 790,195; 704,641; 853,414; 886,784 and899,156. It is also known to modify such catalysts with a titaniumcompound, for example, to render the catalyst capable of producingpolyolefins having increased melt index (i.e., lower average molecularweight) or to increase the stress crack resistance of the producedpolyolefin. Catalysts of this type are described in, for example, U.S.Pat. No. 3,622,521 to Hogan et al and U.K. published patentspecification Nos. 1,334,662 and 1,326,167.

U.S. Pat. No. 3,351,623 to Walker et al discloses a catalyst forpolymerizing ethylene at a temperature in the range of 275° to 335° F.,i.e., under solution form process conditions, the catalyst being onewhich forms on mixing (1) an oxide component selected from the groupconsisting of silica; mixtures of silica and alumina containing up to 25wt. % alumina; and mixtures of chromium oxide and at least one materialselected from the group consisting of silica, alumina, zirconia andthoria, at least part of the chromium oxide being in the hexavalentstate at the initial contacting of the monomer therewith, (2) an organometallic component of formula R_(x) M where R is selected from the groupconsisting of hydrogen and alkyl, aryl, cycloalkyl, alkoxy radicals andcombinations of these radicals containing up to 12 carbon atoms, atleast one R group being a hydrocarbon radical; M is selected from thegroup consisting of aluminum, gallium, indium, thallium, zinc, boron,lead, lithium, zirconium, cobalt, magnesium and tin; and x is an integerequal to the valence of M, and (3) a vanadium component selected fromthe group consisting of vanadium chelates and vanadyl chelates. Walkeret al seek to produce a catalyst that permits the use of highpolymerization temperatures to obtain relatively low melt index polymer.

Downs, U.S. Pat. No. 4,368,302, discloses a modified alpha-olefincatalyst composition used in preparing polymers of ethylene orcopolymers of ethylene and higher alpha-olefins, having a relativelybroad molecular weight distribution, as evidenced by relatively highvalues of melt flow ratio (MFR), referred to by Downs as melt indexratio (MIR). The catalyst composition of Downs is prepared by supportingchromium oxide on a refractory oxide support material, adding at leastone tetravalent titanium compound and heating to activate the product.The monomer is contacted with the active polymerization catalyst in thepresence of one or more organometallic compounds, e.g.,triethylaluminum. The refractory oxide support has a mean particlediameter of about 20-150 microns (μ).

U.K. patent application No. 2,023,153 to Eve discloses an ethylenepolymerization catalyst comprising: (A) a heat activated supportedchromium oxide catalyst, and (B) a catalyst modifier comprising amagnesium compound, such as a dialkyl magnesium, preferably dibutylmagnesium, and a commercially available product believed to comprise amixture of dibutyl magnesium, dialkyl magnesium and butyl-alkylmagnesium. The polymerization process, conducted in the presence of thiscatalyst, produces polyethylene or ethylene copolymers having broadmolecular weight distribution and low melt index. The quantity of thecatalyst modifier is such that the atomic ratio of magnesium to chromiumin the modified catalyst system is 0.01:1 to 100:1, preferably, 0.01:1to 10:1.

Stricklen et al., U.S. Pat. No. 4,374,234, disclose a silica-supportedchromium catalyst to which is added up to 5 ppm of an aluminumalkyl ordihydrocarbomagnesium compound in order to reduce the induction periodand increase catalyst activity. The addition of the aluminum or themagnesium compounds has only a modest effect on polymer properties.

When polymers of ethylene or copolymers of ethylene and of one or moreC₃ -C₁₀ alpha-olefin useful for the blow-molding applications, e.g., forthe HIC market, are produced using the above-identified prior artcatalysts in a fluid bed reactor, the reaction must either be conductedat relatively high temperatures of about 108°-110° C., approachingmelting or sintering temperatures of the product resin, or with arelatively high level of oxygen intentionally added to the reactor(oxygen add-back) to produce a resin having the desired high load meltindex (HLMI).

The operation of fluid bed reactor at the high temperature requires veryelaborate control apparatus to prevent fusion of the resin particles,particularly the small size resin particles (fines), e.g., particles ofthe size less than 74 microns (μ) in the reactor, and particularlydownstream of the reactor, e.g., in the recycle compressor. Since atypical resin produced with such a catalyst begins to melt andagglomerate at about 113° C., even minute temperature excursions abovethe reactor operating temperature can cause severe fines agglomeration,referred to in the art as fouling, in the process equipment downstreamof the reactor, e.g., compressor and heat exchangers, due to melting andfusion of the fines.

As is known to those skilled in the art, the fluid bed alpha-olefinpolymerization reactor system utilizes a recirculation compressor torecycle the fluidizing gas and unreacted monomers, if any, to the fluidbed reactor. The recycled fluidizing gas also contains at least someentrained polymer particles. The system also utilizes heat exchangers,usually located downstream of the compressor, to remove substantialamounts of exothermic heat generated during the reaction (e.g., seeGoeke et al, U.S. Pat. No. 4,302,565). The compressor inherentlyincreases the temperature of the gas stream compressed therein. Sincethe temperature gradient between the fluid bed reactor operatingtemperature and the melting temperature of the resin is very small(e.g., about 7°-9° C.), even a small elevation of the temperature of thefluidizing gas may cause fusion and agglomeration of the polymerparticles in the compressor. If the temperature of the recycle gas isincreased to the level wherein the fusion of the polymer particles takesplace, the polymer particles will also tend to fuse and agglomerate inthe heat exchangers and on the distribution plate of the reactorutilized to provide a uniform fluidized bed of particles therein.

The danger of polymer particles' fusion is exacerbated by the presenceof catalyst particles in the polymer particles. As is known to thoseskilled in the art, the product polymer particles, also referred toherein as resin particles, produced in the polymerization reactorsystem, contain minute amounts of the catalyst particles which are notremoved therefrom because the content thereof in the resin is so smallas to render the removal of the catalyst particles unnecessary. Thecatalyst is inactivated when the resin is removed from the reactor.However, the resin in the reactor system contains catalytically-activecatalyst particles. These catalytically active catalyst particlescontinue to promote the alpha-olefin polymerization reaction which isexothermic in nature, thereby producing additional amounts of heat. Thisheat, together with the inherent increase of the recycle gas temperaturein the recycle compressor, combines to dangerously decrease the safetytemperature margin between the reactor operating temperature and theresin fusion temperature. The polymer particles most susceptible tofusion are polymer fines. If fusion and agglomeration of polymerparticles, particularly polymer fines, takes place in the recyclecompressor, heat exchangers, on the distribution plate and/or any otherparts of the polymerization system, the process efficiency issubstantially decreased, and, ultimately, the fluidized bed system mayhave to be shut down to remove the fused polymer particles.

As mentioned above, the HLMI of the resin may also be increased byincreasing the amount of oxygen added to the reactor, e.g., see Dalig etal. KHIMIYA I TEKNOLOGIYA POLYMEROV, Vol. 23, No. 4 (1961), Ermakov etal, "Transfer Processes During Polymerization of Ethylene on a ChromiumOxide Catalyst. II. The Role of Impurities In Transfer Reactions",KINETICS AND CATALYSIS (USSR), Vol. 10, No. 333 (1969). However, theincreased oxygen content in the reactor may promote the formation ofpolymer fines, which are most likely to fuse in the reactor system. Thealpha-olefin fluidized bed reactors, unlike fluidized bed reactors usedin different chemical processes, e.g., fluid catalytic cracking, do notusually utilize fines removing equipment, such as cyclones or filters,because, it is feared that such equipment may provide additional sitesfor fines to fuse and agglomerate. Thus, any polymer fines produced inthe polymerization system tend to remain in the reactor loop.Accordingly, the use of relatively high amounts of oxygen to obtainresins of a desired HLMI may also lead to the fouling of the processequipment, such as compressor and heat exchangers, and, ultimately, tothe shut-down of the reactor system.

A catalyst composition capable of producing alpha-olefin polymer resinsat a relatively low polymerization temperature and at relatively lowlevels of oxygen is disclosed in a co-pending patent application ofHsieh et al, Ser. No. 939,323, filed on Dec. 8, 1986, the entirecontents of which are incorporated herein by reference. That catalystcomposition comprises a chromium compound and a tetravalent titaniumcompound supported on a refractory oxide support. The catalyst particlesare substantially non-spherical and irregular in shape and they have arelatively broad particle size distribution range. The support has atleast 75% of its pore volume in pores of about 200 to about 500 Angstromdiameter. The catalyst composition is synthesized with at least onetetravalent titanium compound; with CrO₃ or any compound of chromiumcalcinable thereto; and it is activated by heating the catalyst for atleast about 10 hours at a temperature of at least about 800° C.

I now found that chromium-containing catalyst compositions can bemodified to produce alpha-olefin resins having high HLMI at temperaturesand at added oxygen levels lower than those possible withpreviously-available catalyst compositions.

Accordingly, it is a primary object of this invention to provide animproved polymerization catalyst which produces polymers of HLMIsuitable for the blow molding applications at substantially loweroperating temperatures than was heretofore possible with other similarcatalysts.

It is an additional object of the present invention to provide analpha-olefin polymerization process which produces polymer resins ofethylene and/or C₃ -C₁₀ alpha-olefins, having such an HLMI that they aresuitable for the blow molding applications, at lower temperatures thanwas heretofore thought possible.

Additional objects of the invention will become apparent to thoseskilled in the art from the following specification and the attachedclaims.

SUMMARY OF THE INVENTION

A modified alpha-olefin polymerization catalyst composition, alsoreferred to herein as a catalyst complex, is obtained by combining achromium compound-containing catalyst composition with a catalystmodifier which is an oxide of an element of Group II A of the PeriodicChart of the Elements, published by the Fisher Scientific Company,Catalog Number 5-702-10, containing at least about 0.5% by weight ofwater. Preferably, the modifier has the particle size distribution rangecompatible with that of the catalyst composition. The modified catalystcomposition is used to polymerize C₂ -C₁₀ alpha-olefins to form resinshaving higher HLMI values than those produced with the catalystcomposition devoid of the modifier. The resins having high HLMI valuescan be prepared with the catalyst composition of the invention evenwithout oxygen intentionally added to the polymerization reactor.

BRIEF DESCRIPTION OF THE DRAWING

The FIGURE is a schematic representation of a fluidized gas phasereaction apparatus which can be used to prepare polymers in the presenceof the catalyst of this invention.

DETAILED DESCRIPTION OF THE INVENTION

The modified catalyst composition is prepared by combining the modifierwith any supported chromium-containing alpha-olefin polymerizationcatalyst composition in any suitable manner. Suitable catalystcompositions of this type are supported chromium catalysts disclosed,for example, by Hogan et al, U.S. Pat. No. 3,622,521, or Levine et al,U.S. Pat. No. 4,011,382, and Hogan et al, U.S. Pat. No. 2,825,721, theentire contents of all of which are incorporated by reference herein.Suitable chromium compounds include chromium nitrate, chromiumcarbonate, chromium acetate, ammonium chromate, chromium fluoride,tertiary butyl chromate, chromium acetylacetonate and chromium sulphate.Water soluble compounds of chromium, such as CrO₃, are the preferredcompounds used to deposit the chromium compound on the refractory oxidesupport from a solution thereof. However, organic solvents-solublechromium compounds may also be used.

The polymerization catalyst composition also preferably contains atleast one titanium compound. The titanium compound or compounds whichmay be used include all titanium compounds which are oxidizable to TiO₂under the activation conditions employed, and include those disclosed inU.S. Pat. No. 3,622,521 and Netherlands patent application No. 72-10881(the disclosure of both of which are incorporated herein by reference).These compounds include those having the structures

(R₁)_(n) Ti(OR₁)_(m), (R₂ O)_(m) Ti(OR₁)_(n), (R₁)_(n) TiX_(m), (RO)_(n)TiX_(m) and TiX₄

where

m is 1, 2, 3, or 4, n is 0, 1, 2 or 3 and m+n=4; R₁ is a C₁ to C₁₂alkyl, aryl or cycloalkyl group, and combinations thereof, such asaralkyl and alkaryl groups, R₂ is R₁, a cyclopentadienyl group, C₂ toC₁₂ alkenyl groups, such as ethenyl, propenyl, isopropenyl or butenyl,and

X is chlorine, bromine, fluorine or iodine.

The titanium compounds thus include titanium tetrachloride, titaniumtetraisopropoxide and titanium tetrabutoxide. The titanium compounds areconveniently deposited on the support from a hydrocarbon solventsolution thereof.

The amount of chromium and titanium compounds on the catalyst willdepend on a particular catalyst composition used in a particular processcontext.

The modifier may be blended with the catalyst composition outside of thepolymerization reactor after both, the catalyst composition and themodifier, are separately prepared by their respective processes.Alternatively, the catalyst modifier may be introduced into thepolymerization reactor while the catalyst composition is introducedseparately into the polymerization reactor. However, in a preferredembodiment, the catalyst modifier is dry blended with the catalystcomposition outside of the polymerization reactor and the thus-producedmodified catalyst composition is then used in the polymerizationreactor. It is important that the modifier contain at least some waterin the form of moisture. Thus, the modifier must contain at least about0.5, preferably at least about 1, more preferably at least about 1.5,and most preferably about 2% by weight of water. The water may beincorporated into the modifier by any convenient means, such as exposingthe modifier for a sufficient period of time to ambient atmosphere toproduce the modifier containing the aforementioned amounts of water,contacting the modifier with moisture-containing gaseous stream or withliquid water, or by any other convenient means. Preferably, the water isincorporated into the modifier by equilibrating the required amount ofthe modifier in an ambient atmosphere containing moisture. It is alsoimportant that the particle size distribution range of the modifier becompatible with that of the catalyst composition. Thus, the particlesize distribution of the modifier should be compatible with that of thecatalyst composition with which it is combined but it need not beidentical thereto. The term "compatible" is used herein to designatesuch a particle size distribution which enables the modifier to bereadily blended with the catalyst composition. The amount of thecatalyst modifier is about 1 to about 15, preferably about 3 to about 9and most preferably about 5 to about 7% by weight based on the totalweight of the catalyst composition. Suitable catalyst modifiers areberyllium oxide, magnesium oxide, calcium oxide, strontium oxide, bariumoxide, and radium oxide, preferably calcium or magnesium oxide, and mostpreferably magnesium oxide. It is also possible to use a mixture of theaforementioned catalyst modifiers so long as the total amount of the sumof all of the modifiers is within the aforementioned weight percentageamounts. In a preferred embodiment, however, only one catalyst modifieris used to obtain the modified catalyst composition of the presentinvention.

Most Preferred Embodiment

In the most preferred embodiment, the modified catalyst composition ofthe present invention is prepared with the catalytic composition ofHsieh et al, described and claimed in a co-pending patent application,Ser. No. 939,323, filed on Dec. 8, 1986 (hereinafter referred to as "thecatalyst composition of Hsieh et al"). Accordingly, for the purposes ofsimplification and exemplification, the invention will now be describedin conjunction with the most preferred embodiment of the invention.However, it will be apparent to those skilled in the art that theinvention is not limited to this preferred embodiment since the modifiedcatalyst composition of the invention can be prepared with anychromium-containing alpha-olefin polymerization catalyst (i.e., any"Phillips catalyst").

The catalyst of Hsieh et al is prepared in a process comprising thesteps of:

1. drying or dehydrating the refractory oxide support material at atemperature of about 100° to about 500° C., preferably about 150° toabout 300° C., and most preferably at about 200° C. for at least about14 hours (hrs), preferably at least about 16 hours, and most preferablyfor about 16 hrs in a nitrogen atmosphere;

2. if required, depositing a chromium compound onto the dried supportand drying the chromium-containing support at 200° C. for at least about14 hours;

3. contacting the thus-obtained catalyst intermediate with at least onetetravalent titanium compound; and

4. purging the flammable components with nitrogen, preferably at about325° C., and subsequently thermally activating the resulting solid, inan air atmosphere, at a temperature of at least about 800° C.,preferably at least about 820° C., more preferably at about 820° toabout 870° C., and most preferably at about 825° C. for at least about10 hrs; preferably at least about 14 hrs, more preferably for at leastabout 16 hrs, and most preferably for about 16 hrs, to obtain an activecatalyst composition.

In a preferred embodiment of the Hsieh et al catalyst synthesis, therefractory oxide support has a chromium compound deposited thereon priorto the drying or dehydration thereof in step 1. Thus, in such anembodiment, step 2., above, is omitted.

The refractory oxide support material used to synthesize the Hsieh et alcatalyst can be any support known in the art, such as silica,silica-alumina, silica-titania, alumina, zirconia or thoria, so long asit produces catalyst particles which are non-spherical and irregular inshape and of such size that the diameter of the catalyst particles isabout 5 to about 250 microns (μ), preferably about 10 to about 200μ, asdetermined by Scanning Electron Microscopy (SEM). The majority of theparticles of the Hsieh et al catalyst must have relatively largeparticle diameters. Thus, the catalyst has a median particle diameter ofabout 50 to about 100, preferably about 60 to about 80 and mostpreferably about 75μ, as determined by volume distribution using aHIAC-ROYCO PA-720 particle size analyzer, and it has the followingparticle size distribution:cumulative volume distribution--10 to 90% ofthe particles have diameters of about 28 to about 150, preferably about30 to about 125 and most preferably about 37 to about 117μ, also asdetermined in a HIAC-ROYCO particle size analyzer. Additionally, atleast 75% of the refractory oxide support's pore volume is in the poresof about 200 to about 500 Angstrom diameter, preferably at least 80% ofits pore volume is in the pores of about 250 to about 450 Angstromdiameter, and most preferably at least 85% of its pore volume is in thepores of about 250 to about 450 Angstrom diameter. The most preferredrefractory oxide support material used for the Hsieh et al catalyst issilica containing about 0.18% by weight of chromium trioxide (CrO₃) andhaving about 88% of its pore volume in the pores of about 250 to about450 Angstrom diameter, and particle diameter of about 10 to about 200microns, available commercially from Joseph Crosfield & Sons Ltd.,Applied Silicas Division, Warrington, England, under the tradename ofSD-186.

The chromium compound and the tetravalent titanium compound are usuallydeposited on the support from the solutions thereof and in suchquantities as to provide, after the activation step, the desired levelsof chromium (Cr) and titanium (Ti) in the catalyst. The order ofaddition of the chromium and titanium compounds to the support is notcritical, provided that such compounds are added before the activationof the catalyst and the support is dried before the titanium compound isadded thereto. However, it is preferred to deposit the chromium compoundonto the support before the titanium compound.

The chromium compound is CrO₃ or any compound of chromium which iscalcinable thereto under the activation conditions employed during theHsieh et al catalyst synthesis. At least a portion of the chromium inthe supported, activated catalyst must be in the hexavalent state.Chromium compounds, other than CrO₃, which may be used are thosedisclosed above as being suitable for the preparation of the chromiumcompound-containing alpha-olefin polymerization catalysts, such as thoseof U.S. Patents to Hogan et al., U.S. Pat. Nos. 2,825,721 and 3,622,521,and include chromium nitrate, chromium carbonate, chromium acetate,ammonium chromate, chromium chloride, tertiary butyl chromate, chromiumacetylacetonate, and chromium sulphate.

Water soluble compounds of chromium, such as CrO₃, are the preferredchromium compounds used in the Hsieh et al catalyst synthesis. However,organic solvents-soluble chromium compounds may also be used.

After the chromium compound is deposited on the support, the chromiumcompound-containing support is dehydrated or dried at the aforementionedconditions.

Subsequently, the dehydrated catalyst intermediate is contacted with atitanium compound or compounds. Suitable titanium compounds which may beused are also those disclosed above as being suitable for thepreparation of the chromium and titanium compound-containingalpha-olefin polymerization catalysts.

The most preferred titanium compound is tetraisopropyltitanate. Theamount of the titanium compound used is such that there is an excess ofthe moles of titanium (calculated as elemental titanium) with respect tothe moles of chromium (calculated as elemental chromium) in the catalystcomposition. Thus, the titanium (calculated as elemental titanium) ispresent in the final catalyst product, with respect to the chromium(calculated as elemental chromium), in a molar ratio of about 0.5 toabout 180, and preferably about 4 to about 35.

After the titanium compound is deposited on the catalyst, the resultingsolid material is activated at the aforementioned conditions.

The activated supported catalyst of Hsieh et al contains, based on thecombined weight of the support and the chromium and titanium therein,about 0.05 to about 3.0, preferably about 0.10 to about 0.25, morepreferably about 0.13 to about 0.18 and most preferably about 0.15 toabout 0.18 weight percent of chromium (calculated as elementalchromium), and about 1.5 to about 9.0, preferably about 3.0 to about 7.0and most preferably about 3.7 to about 6.0 weight percent of titanium(calculated as elemental titanium).

The resulting activated catalyst composition is then combined with themodifier and the resulting modified catalyst composition, when used,preferably in the presence of small amounts of hydrogen, to polymerizealpha-olefins is extremely effective in preparing alpha-olefin polymerssuitable for blow-molding applications, such as for the HIC market,i.e., alpha-olefin polymers having relatively low die swellcharacteristics which render them suitable for the preparation ofhousehold and industrial containers, e.g., household cleaners bottlesand possibly even products requiring higher HLMI values, such as milkbottles. Since it is preferred that the particle size distribution ofthe modifier be compatible with that of the catalyst composition, themodifier combined with the Hsieh et al catalyst composition has theparticle diameter of about 5 to about 250, preferably about 10 to about200, and most preferably about 45 to about 180 microns.

The modified catalyst composition is used to polymerize either ethylenealone or ethylene in conjunction with higher monomers, such as any oneof C₃ -C₁₀ alpha-olefins, e.g., 1-butene, 1-pentene, 1-hexene,4-methyl-1-pentene, 1-heptene or 1-octene, preferably 1-butene,1-pentene, 1-hexene or 4-methyl-1-pentene and most preferably 1-hexene.The polymerization reaction is preferably carried out in a fluid bedreactor, as described below, and substantially in the absence ofcatalyst poisons, such as moisture, carbon monoxide and acetylene, witha catalytically effective amount of the modified catalyst composition ata temperature and pressure conditions sufficient to initiate thepolymerization reaction.

In order to achieve density ranges of about 0.915 to about 0.965 g/cc inthe copolymers, which includes the range of about 0.947 to about 0.959g/cc required for HIC applications, it is necessary to copolymerizeenough of the C₃ or higher comonomers with ethylene to achieve a levelof about 0.1 to about 25 mole percent of the C₃ to C₁₀ comonomer in thecopolymer. The amount of comonomer needed to achieve this result willdepend on the particular comonomer(s) being employed. Further, thevarious intended comonomers have different reactivity rates, relative tothe reactivity rate of ethylene, with respect to the copolymerizationthereof with the catalysts of the present invention. Therefore, theamount of comonomer used in the stream of monomers fed to the reactorwill also vary depending on the reactivity of the comonomer.

In the most preferred embodiment, the modified catalyst composition ofthe present invention is used to polymerize 1-hexene and ethylene in afluid bed process to obtain polymers having the density of about 0.947to about 0.959 g/cc, which are especially suitable for HIC applications.

The modified catalyst composition of this invention is especiallysuitable for producing polymers of C₂ -C₁₀ alpha-olefins in a gas phase,fluid bed reactor in the presence of about 0 to about 0.35 ppmw ofoxygen, calculated on the basis of ethylene feed because it makespossible the operation of the reactor at a temperature not higher thanabout 108° C., preferably not higher than about 105° C., to producepolymer resins having HLMI values of about 25-60, preferably about30-50, and most preferably about 40-50. As is known to those skilled inthe art, heretofore-used Phillips catalysts required gas phase fluid bedreactor temperatures of about 108° C.-110° C. to produce polymer resinshaving the aforementioned HLMI values. The resins must have these HLMIvalues to render them suitable for processing in blow-molding machinesto produce products for the HIC market, such as bottles having highenvironmental stress crack resistance. Since the resins begin to fuseand agglomerate at about 113° C., the reactor operation at about108°-110° C. may, and occasionally does, cause at least partial fusionof fine resin particles downstream of the fluidized bed reactor, e.g.,in the compressor used to recirculate the reactor gases. In prior art,the HLMI of the resin was also increased by increasing the amount ofoxygen or oxygen-containing gas (oxygen add-back), such as air, added tothe gas phase, fluid bed reactor. While the reactor operation with thehigher oxygen add-back increased the HLMI to the desired level, it alsocreated additional quantities of resin fines which increased the risk offouling of the process equipment, such as the gas recirculationcompressor and heat exchangers.

The modified catalyst composition of this invention substantiallydecreases, if not completely eliminates, the danger of process equipmentfouling because, at a given level of oxygen introduced into the reactor,it enables the operation of the polymerization reaction at asubstantially lower temperature than was heretofore thought possible toproduce polymer resins having the desired HLMI values. Thus, the safetymargin between the reactor operating temperature and the resin fusiontemperature is increased, thereby reducing, if not substantiallyeliminating, the danger of equipment fouling by the fused (melted) resinparticles. It is also notable that the modified catalyst composition ofthe invention is capable of producing polymer resins having theaforementioned HLMI values even in the substantial absence of addedoxygen. This is especially surprising since the catalyst composition ofHsieh et al required added oxygen in most instances to produce resins ofthe required HLMI, as set forth in the aforementioned application ofHsieh et al.

The operation of the fluidized bed reactor with the modified catalystcomposition of the invention also produces substantially reduced amountsof polymer fines, as compared to the operation of the reactor atcomparable conditions with the previously-available Phillips catalystcompositions. Since the polymer fines are most likely to fuse when thetemperature safety margin is decreased, e.g., in the compressor or dueto unanticipated temperature surges, the low level of fines providesadditional protection against polymer fusion and agglomeration in thepolymerization system.

Additionally, the feedability characteristics of the preferred modifiedcatalyst composition of this invention, prepared with the catalystcomposition of Hsieh et al, are improved in comparison to some prior artPhillips catalysts, as evidenced by the lack of agglomeration of thecatalyst particles in the feeder of the fluid bed polymerizationreactor. Some prior art catalyst compositions sometimes tended to clogup the feeding mechanism of the fluid bed reactor necessitating frequentprocess interruptions. The preferred modified catalyst composition ofthis invention exhibits substantially no tendency to clog up the fluidbed reactor feeding mechanism.

The modified catalyst composition of the preferred embodiment of theinvention also has very good fluidizing properties, as evidenced by asubstantial lack of sheeting in the reactor. This is especiallysurprising and unexpected since the prior art taught thatspherically-shaped catalyst particles are easier to fluidize thanirregularly shaped catalyst particles (see Hamer et al, U.S. Pat. No.4,293,673). However, I found that the modified catalyst compositionprepared with the catalyst composition of Hsieh et al, havingnon-spherical, irregularly-shaped particles, has very good fluidizingcharacteristics, in addition to the advantages detailed above.

A fluidized bed reaction system which can be used in the practice of theprocess of the present invention is illustrated in FIG. 1. Withreference thereto, a reactor 10 consists of a reaction zone 12 and avelocity reduction zone 14.

The reaction zone 12 comprises a bed of growing polymer particles,formed polymer particles and a minor amount of catalyst particlesfluidized by the continuous flow of polymerizable and modifying gaseouscomponents in the form of make-up feed and recycle gas through thereaction zone. To maintain a viable fluidized bed, the mass gas flowrate through the bed must be above the minimum flow required forfluidization, and preferably from about 1.5 to about 10 times G_(mf),and more preferably from about 3 to about 6 times G_(mf). The termG_(mf) is used herein in the accepted form as the abbreviation for theminimum mass gas flow required to achieve fluidization, see C. Y. Wenand Y. H. Yu, "Mechanics of Fluidization", Chemical Engineering ProgressSymposium Series, Vol. 62, p. 100-111 (1966).

The modified catalyst composition used in the fluidized bed ispreferably stored for service in a reservoir 32 under a blanket of a gaswhich is inert to the stored material, such as nitrogen or argon.

Fluidization is achieved by a high rate of gas recycle to and throughthe bed, typically on the order of about 50 times the rate of feed ofmake-up gas. The fluidized bed has the general appearance of a densemass of viable particles in possible free-vortex flow as created by thepercolation of gas through the bed. The pressure drop through the bed isequal to or slightly greater than the mass of the bed divided by thecross-sectional area. It is thus dependent on the geometry of thereactor.

Make-up gas is fed to the bed at a rate equal to the rate at whichparticulate polymer product is withdrawn. The composition of the make-upgas is determined by a gas analyzer 16 positioned above the bed. The gasanalyzer determines the composition of the gas being recycled, and thecomposition of the make-up gas is adjusted accordingly to maintain anessentially steady state gaseous composition within the reaction zone.

To insure complete fluidization, the recycle gas and, where desired, apart of the make-up gas are returned to the reacter at point 18 belowthe bed. A gas distribution plate 20 is positioned above the point ofreturn to aid in the fluidization of the bed.

The portion of the gas stream which does not react in the bedconstitutes the recycle gas which is removed from the polymerizationzone, preferably by passing into a velocity reduction zone 14 above thebed where entrained particles are given an opportunity to return to thebed. No fines removal equipment, such as cyclones or filters, is used inthe fluidized bed system because of the danger of fines fusing therein.

The recycle gas is compressed in a compressor 25 and then passed througha heat exchanger 26 wherein it is stripped of heat of reaction before itis returned to the bed. By constantly removing heat of reaction, nosignificant temperature gradient appears to exist within the upperportion of the bed. A temperature gradient will exist in the bottom ofthe bed in a layer of about 6 to 12 inches, between the temperature ofthe inlet gas and the temperature of the remainder of the bed. Thus, ithas been observed that the bed acts to almost immediately adjust thetemperature of the recycle gas above this bottom layer of the bed zoneto make it conform to the temperature of the remainder of the bed,thereby maintaining itself at an essentially constant temperature understeady state conditions. The recycle is then returned to the reactor atits base 18 and to the fluidized bed through distribution plate 20. Thecompressor 25 can also be placed upstream of the heat exchanger 26.

The distribution plate 20 plays an important role in the operation ofthe reactor. The fluidized bed contains growing and formed particulatepolymer particles as well as catalyst particles. As the polymerparticles are hot and possibly active, they must be prevented fromsettling, for if a quiescent mass is allowed to exist, any activecatalyst contained therein may continue to react and cause fusion.Diffusing recycle gas through the bed at a rate sufficient to maintainfluidization at the base of the bed is, therefore, important. Thedistribution plate 20 serves this purpose and may be a screen, slottedplate, perforated plate, a plate of the bubble cap type, or any similarplate known in the art. The elements of the plate may all be stationary,or the plate may be of the mobile type disclosed in U.S. Pat. No.3,298.792, the entire contents of which are incorporated herein byreference. Whatever the design of the plate, it must diffuse the recyclegas through the particles at the base of the bed to keep them in afluidized condition, and also serve to support a quiescent bed of resinparticles when the reactor is not in operation. The mobile elements ofthe plate may be used to dislodge any polymer particles entrapped in oron the plate.

Hydrogen may be used in the polymerization reaction of the presentinvention in such amounts that the ratio of hydrogen to ethylene isbetween about 0 to about 2.0 moles of hydrogen per mole of the ethylenemonomer in the gas stream. Any gas inert to the catalyst and reactantscan also be present in the gas stream.

It is essential to operate the fluid bed reactor at a temperature belowthe sintering temperature of the polymer particles. To ensure thatsintering will not occur, operating temperatures below the sinteringtemperature are desired. For the production of ethylene copolymers anoperating temperature of about 30° to 105° C. is preferred, and atemperature of about 75° to 103° C. is most preferred. Temperatures ofabout 75° to 90° C. are used to prepare polymer products having adensity of about 0.90 to 0.92 gms/cc, temperatures of about 80° to 100°C. are used to prepare products having a density of about 0.92 to 0.94gms/cc, and temperatures of about 90° to 105° C. are used to prepareproducts having a density of about 0.94 to 0.96 gms/cc.

The fluid bed reactor is operated at pressures of up to about 1000 psi,and is preferably operated at a pressure of from about 150 to 350 psi,with operation at the higher pressures in such ranges favoring heattransfer since an increase in pressure increases the unit volume heatcapacity of the gas.

The modified catalyst is injected into the bed at a rate equal to itsconsumption at a point 30 which is above the distribution plate 20. Theinjection of the catalyst at a point above the distribution plate is animportant feature of this process. Since the catalyst used in thepractice of this invention is highly active, the injection of the fullyactivated catalyst into the area below the distrubition plate may causepolymerization to begin in that area and eventually cause plugging ofthe distribution plate. Injection into the viable bed, instead, aids indistributing the catalyst throughout the bed and tends to preclude theformation of localized spots of high catalyst concentration which mayresult in the formation of "hot spots".

A gas which is inert to the catalyst, such as nitrogen or argon, is usedto carry it and/or any additional modifier compound or compounds thatare needed, into the bed.

The production rate of the bed is controlled by the rate of the catalystinjection. The production rate may be increased by simply increasing therate of catalyst injection and decreased by reducing the rate ofinjection thereof.

Since any change in the rate of catalyst injection will change the rateof generation of the heat of reaction, the temperature of the recyclegas is adjusted upwards or downwards to accommodate the change in therate of heat generation. This ensures the maintenance of an essentiallyconstant temperature in the bed. It will be apparent to those skilled inthe art that complete instrumentation of both the fluidized bed and therecycle gas cooling system is necessary to detect any temperature changein the bed so as to enable the operator to make a suitable adjustment inthe temperature of the recycle gas.

Under a given set of operating conditions, the fluidized bed ismaintained at essentially a constant height by withdrawing a portion ofthe bed as product at a rate equal to the rate of formation of theparticulate polymer product. Since the rate of heat generation isdirectly related to product formation, a measurement of the temperaturerise of the gas across the reactor (the difference between inlet gastemperature and exit gas temperature) is determinative of the rate ofparticulate polymer formation at a constant gas velocity.

The particulate polymer product is preferably continuously withdrawn ata point 34 at or close to the distribution plate 20 and in suspensionwith a portion of the gas stream which is vented before the particulatessettle to preclude further polymerization and sintering when theparticles reach their ultimate collection zone. The suspending gas mayalso be used, as mentioned above, to drive the product of one reactor toanother reactor.

The particulate polymer product is conveniently and preferably withdrawnthrough the sequential operation of a pair of timed valves 36 and 38defining a segregation zone 40. While valve 38 is closed, valve 36 isopened to emit a plug of gas and product to the zone 40 between it andvalve 36 which is then closed. Valve 38 is then opened to deliver theproduct to an external recovery zone. Valve 38 is then closed to awaitthe next product recovery operation.

Finally, the fluidized bed reactor is equipped with an adequate ventingsystem to allow venting the bed during start up and shut down. Thereactor does not require the use of stirring means and/or wall scrapingmeans.

The highly active supported catalyst of this invention yields a fluidbed product having an average particle size between about 0.005 to about0.07 inches and preferably about f0.02 to about 0.04 inches.

The feed stream of a gaseous monomer, with or without inert gaseousdiluents, is fed into the reactor at a space time yield of about 2 to 10pounds/hour/cubic foot of bed volume.

The use of the modified catalyst of the present invention in thefluidized bed polymerization process described above or in any othersuitable olefin polymerization process, e.g., slurry process, producespolymers in which the net bottle weight and HLMI characteristics can besimultaneously controlled without adversely affecting other propertiesof the polymers, such as strength thereof. The modified catalyst of theinvention can be used to produce both linear low density polyethylene(LLDPE) and high density polyethylene (HDPE), but it is preferably usedto produce HDPE.

The following Examples further illustrate the essential features of theinvention. However, it will be apparent to those skilled in the art thatthe specific reactants and reaction conditions used in the Examples donot limit the scope of the invention.

The properties of the polymers produced in the Examples were determinedby the following test methods:

    ______________________________________                                        Density         ASTM D-1505 - A plaque is made                                                and conditioned for one hour at                                               100° C. to approach equilibrium                                        crystallinity. Measurement for den-                                           sity is then made in a density grad-                                          ient column; reported as gms/cc.                              Melt Index (MI), I.sub.2                                                                      ASTM D-1238 - Condition E -                                                   Measured at 190° C. - reported as                                      grams per 10 minutes.                                         High Load Melt Index                                                                          ASTM D-1238 - Condition F -                                   (HLMI), I.sub.21                                                                              Measured at 10 times the weight                                               used in the melt index test above.                             Melt Flow Ratio (MFR) =                                                                       ##STR1##                                                     Weight Swell or Conducted on an Impco B-13 Blow                               Net Bottle Weight                                                                             Molding Machine. The machine is                                               calibrated on a standard material,                                            Marlex 5502, to give a 72.0 ± 1.0 g                                        bottle at a parison drop time of                                              2.0 ± 0.03 sec. The test material is                                       then introduced, and the machine                                              readjusted to a 2.0 sec parison drop                                          time. The measured weight is re-                                              ported after correcting for the dif-                                          ference between the actual standard                                           weight and 72.0 g.                                            Annular Die Swell (gr)                                                                        Measured by weighing 252 centi-                                               meters (cm) of a tube extruded at                                             shear rate of 2000 l/sec through an                                           annular die having a tapered en-                                              trance angle of 24.4°; die lend                                        length of 3.05 mm; die gap of 0.505                                           mm and average die diameter of                                                5.16 mm.                                                      ______________________________________                                    

EXAMPLE 1 (Catalyst Synthesis)

A catalyst composition of Hsieh et al was prepared in the followingmulti-step procedure.

Step 1 (Drying)--A chromium oxide coated silica (0.18% wt. Cr,calculated as chromium trioxide) having about 88% of its pore volume inthe pore sizes of between 250 and 450 Angstroms (Crosfield's SD-186) wasintroduced into a fluid-bed drying vessel maintained under nitrogen (N₂)at an ambient temperature and pressure. The temperature of the vesselwas increased at the rate of 25° C./hour until it reached about200°-210° C. The silica was dried in nitrogen atmosphere at about200°-210° C. for 16 hrs. It was then cooled to below 100° C. in 2-4 hrs.

Step 2 (Titanation)--30.0 grams of the product of Step 1 was charged toa 250 ml flask along with 50 ml of dry hexane. 10.5 ml oftetraisopropyltitanate was added at room temperature and the resultingbrown slurry heated to 60°-65° C. under a vigorous nitrogen purge untilthe solid was dry (about 16-24 hrs). The hexane was removed in an oilbath with the temperature set at 65° C. over the period of 16 hrs.

Step 3 (Activation)--Under good fluidization, the product of Step 2 washeated at 25° C./hr until it reached the temperature of 325° C. It wasthen heated at 325° C. for 2 hours in nitrogen; the nitrogen atmospherewas changed to air; the temperature was increased from 325° C. to 825°C. at 100° C./hr; and, it was heated for 16 hrs at 825° C. in dry air.The temperature was then decreased to 350° C. as fast as possible. At350° C., the air was again changed to nitrogen, and the temperature wasdecreased to ambient as fast as possible. The nitrogen purge wasmaintained for 1 hour at ambient temperature. The product was ayellow-orange catalyst which analyzed as follows:

    Cr=0.18%wt.

    Ti=4.0%wt.

EXAMPLE 2 (Preparation of the Modified Catalyst of the Invention)

1.1 grams of the catalyst composition of Example 1 was dry-blended with0.1 grams (g) of magnesium oxide (MgO) powder, obtained from CalgonCorporation, a subsidiary of Merck & Co., Inc., under the name ofMaglite CG-1. The MgO powder, equilibrated to atmospheric moisturecontent, contained about 10% by weight (wt.) of water. The blending wascarried out in a shaken Schlenk tube, for 15 minutes. The MgO particlesize (about 100 to about 170μ) was compatible to that of the catalystcomposition of Example 1 (10-200μ) and the two solids blended well. Theresulting modified catalyst contained 9% by weight (wt %) of MgO.

EXAMPLE 3 (MgO-Containing Modified Catalyst)

One additional sample of the modified catalyst was prepared in themanner substantially similar to that of Example 2, except that asufficient amount of the MgO was used to produce a modified catalystcontaining 15 wt. % of the MgO.

EXAMPLE 4 (Polymerization of Ethylene With 1-Hexene)

Under dry nitrogen, a 2.5-gallon slurry reactor was filled with 4 litersof isobutane. Approximately 1.0 gram of the activated catalyst ofExample 1 was transferred to the reactor. The reactor was then heated to97° C., after which 15 ml of 1-hexene was added thereto. Finallysufficient ethylene was introduced into the reactor to provide ethylenepartial pressure in the reactor of 120 psia. The polymerization wascarried out for about 1 hour at 450 psig total pressure.

EXAMPLES 5-6 (Polymerization of Ethylene With 1-Hexene in the Presenceof the Modified Catalyst)

Ethylene was polymerized substantially in the manner of Example 4 in thepresence of the modified catalyst composition of Examples 2 and 3.

The results of Examples 4-6 are summarized in Table 1, below.

                                      TABLE 1                                     __________________________________________________________________________         Catalyst                                                                           MgO  Catalyst                                                                           HLMI,                                                                              MFR      Annular                                                                             Productivity                               of   Content                                                                            Total                                                                              g/10 min                                                                           (I.sub.21 /                                                                       Density                                                                            Die Swell                                                                           (g PE/g                               Example                                                                            Example                                                                            (wt. %)                                                                            wt. (g)                                                                            (I.sub.21)                                                                         I.sub.2)                                                                          (g/cc)                                                                             (g)   cat/hr)                               __________________________________________________________________________    4    1    0    1.1  11.2 187 0.950                                                                              0.934 634                                   5    2    9    1.1  24.1 142 0.951                                                                              1.018 669                                   6    3    15   1.3  48.3  92 0.952                                                                              1.081 442                                   __________________________________________________________________________

The data of Table 1 indicates that the addition of MgO to the catalystcomposition produces a modified catalyst composition which producespolymers of substantially higher HLMI than similar catalyst compositionswithout the MgO modifier. The data also indicates that the modifiedcatalyst composition produces polymers having acceptable die swellcharacteristics.

EXAMPLES 7-9 (Polymerization of Ethylene and 1-Hexene at LowerTemperatures)

These experiments were conducted to determine if the polymerizationtemperature had an effect on the HLMI-promotion properties of themodified catalyst composition.

The polymerization was conducted in a 2.5 liter (l) slurry reactor with1.25 l of a diluent (specified in Table 2, below), at 90° C. 0.7 g ofthe catalyst and 3 cubic centimeters (cc) of 1-hexene were used in eachexample. The polymerization was conducted substantially in the manner ofExample 4. In Examples 7 and 8, a catalyst composition of Hsieh et al,synthesized substantially in the manner of Example 1 was used. InExample 9, the modified catalyst composition, prepared substantially inthe manner of Example 2 was used.

                  TABLE 2                                                         ______________________________________                                                                                  Pro-                                                      HLMI,  MFR          ductivity                                         MI      g/10 min                                                                             (I.sub.21 /                                                                         Density                                                                              (g PE/g                             Ex.  Diluent  (I.sub.2)                                                                             (I.sub.21)                                                                           I.sub.2)                                                                            (g/cc) cat-hr)                             ______________________________________                                        7    isobutane                                                                              0.0287  10.2   355   0.949  555                                 8    hexane   0.0282  12.1   433   0.951  432                                 9    hexane   0.0268  10.6   395   0.962  448                                 ______________________________________                                    

The data of Table 2 indicates that at 90° C., the catalyst modifier,when used to modify the Hsieh et al catalyst, had substantially noeffect on the HLMI properties of the resin. Accordingly, it is likelythat for every given catalyst composition, there is a thresholdpolymerization temperature necessary for the manifestation of the effectof the catalyst modifier on the HLMI of the polymer resin.

EXAMPLES 10-11 (Effect of Dried Modifier)

These examples were conducted to determine the effect, if any, of thedried MgO on the HLMI properties of the resin produced with a catalystcomposition combined with a dried catalyst modifier.

The catalyst substantially similar to that of Example 1 was used inExample 10. In Example 11, the catalyst of Example 10 was combined with15% wt., based on weight of the catalyst, of MgO, obtained from Calgonunder the name Maglite CG-1, having the particle size of about 45 toabout 150μ, which was dried for 16 hrs at 200° C. under purified flowingnitrogen (N₂). The catalyst of Example 10 and the resulting modifiedcatalyst composition of Example 11 were used to polymerize ethylenehaving a partial pressure of 120 psia with 5 ml of 1-hexene in a 1gallon slurry reactor at 97° C., containing 1 liter of the isobutanediluent, in the presence of 1 gram of the catalyst of Hsieh et al or themodified catalyst of this invention. The results are summarized in Table3.

                  TABLE 3                                                         ______________________________________                                                                    MFR   Den-  Productivity                                         MI     HLMI  (I.sub.21 /                                                                         sity  (g PE/g                               Ex.  Catalyst  (I.sub.2)                                                                            (I.sub.21)                                                                          I.sub.2)                                                                            (g/cc)                                                                              cat-hr)                               ______________________________________                                        10   Hsieh et al                                                                             0.106  16.3  154   0.955 468                                   11   Hsieh et al                                                                             0.069  15.5  225   0.956 419                                        plus 15%                                                                      dried MgO                                                                ______________________________________                                    

The data of Table 3 indicates that the dried MgO promoter producespolymer resins having substantially similar HLMI characteristics as thecatalyst composition of Hsieh et al without the promoter.

It will be apparent to those skilled in the art that the specificembodiments discussed above can be successfully repeated withingredients equivalent to those generically or specifically set forthabove and under variable process conditions.

From the foregoing specification, one skilled in the art can readilyascertain the essential features of this invention and without departingfrom the spirit and scope thereof can adapt it to various diverseapplications.

What is claimed is:
 1. In a supported alpha-olefin polymerizationcatalyst composition comprising a chromium compound, which is CrO₃ orany compound of chromium calcinable to CrO₃ under catalyst activationconditions, an improvement comprising combining the catalyst compositionwith a catalyst modifier which is an oxide of an element of Group IIA ofthe Periodic Chart of the Elements containing at least about 0.5% byweight of water to form a modified catalyst composition.
 2. A modifiedcatalyst composition of claim 1 wherein it additionally comprises atitanium compound which is oxidizable to TiO₂ under catalyst activationconditions.
 3. A modified catalyst composition of claim 2 wherein thecatalyst modifier has particle size distribution range compatible withthat of the supported catalyst composition.
 4. A modified catalystcomposition of claim 3 wherein the supported catalyst composition issubstantially non-spherical and irregular in shape, and which has arelatively broad particle size distribution range.
 5. A modifiedcatalyst composition of claim 4 wherein particle diameter of thecatalyst modifier is about 5 to about 250μ.
 6. A modified catalystcomposition of claim 5 wherein the particle diameter of the catalystmodifier is about 10 to about 200μ.
 7. A modified catalyst compositionof claim 6 wherein the particle diameter of the catalyst modifier isabout 45 to about 180μ.
 8. A modified catalyst composition of claim 7wherein the catalyst modifier contains at least about 1% by weight ofwater.
 9. A modified catalyst composition of claim 8 wherein thecatalyst modifier contains at least about 1.5% by weight of water.
 10. Amodified catalyst composition of claim 9 wherein the catalyst modifiercontains at least about 2% by weight of water.
 11. A modified catalystcomposition of claim 10 which is used in a fluidized bed polymerizationreaction.
 12. A modified catalyst composition of claim 11 wherein thesupported catalyst composition has the particle diameter of about 5 toabout 250μ.
 13. A modified catalyst composition of claim 12 wherein thesupported catalyst composition has the particle diameter of about 10 toabout 200μ.
 14. A modified catalyst composition of claim 13 wherein thesupported catalyst composition is combined with about 10 to about 15% byweight of the catalyst modifier.
 15. A modified catalyst composition ofclaim 14 wherein the supported catalyst composition is combined withabout 3 to about 0% by weight of the catalyst modifier.
 16. A modifiedcatalyst composition of claim 15 wherein the supported catalystcomposition is combined with about 5 to about 7% by weight of thecatalyst modifier.
 17. A modified catalyst composition of claim 16wherein the supported catalyst composition has the following particlesize distribution:cumulative volume distribution--10 to 90% of thecatalyst particles have diameters of about 28 to about 150μ, asdetermined in a HIAC-ROYCO PA-720 particle size analyzer.
 18. A modifiedcatalyst composition of claim 16 wherein the supported catalystcomposition has the following particle size distribution:cumulativevolume distribution--10 to 90% of the catalyst particles have diametersof about 30 to about 125μ, as determined in a HIAC-ROYCO PA-720 particlesize analyzer.
 19. A modified catalyst composition of claim 18 whereinthe supported catalyst composition has the following particle sizedistribution:cumulative volume distribution--10 to 90% of the catalystparticles have diameters of from about 37 to about 117μ, as determinedin a HIAC-ROYCO PA-720 particle size analyzer.
 20. A modified catalystcomposition of claim 19 wherein the supported catalyst composition has amedian particle diameter of about 50 to about 100μ, as determined byvolume distribution using a HIAC-ROYCO PA-720 particle size analyzer.21. A modified catalyst composition of claim 20 wherein the supportedcatalyst composition has a median particle diameter of about 60 to about80μ, as determined by volume distribution using a HIAC-ROYCO PA-720particle size analyzer.
 22. A modified catalyst composition of claim 21wherein the catalyst composition has a median particle diameter of about75μ, as determined by volume distribution using a HIAC-ROYCO PA-720particle size analyzer.
 23. A modified catalyst composition of claim 22wherein the supported catalyst composition is supported on a refractoryoxide support which has at least 75% of its pore volume in the pores ofabout 200 to about 500 Angstrom diameter.
 24. A modified catalystcomposition of claim 23 wherein the supported catalyst composition isactivated for at least about 10 hours at a temperature of at least about800° C.
 25. A modified catalyst composition of claim 24 wherein therefractory oxide support is silica having at least 80% of its porevolume in the pores of about 250 to about 450 Angstrom diameter.
 26. Amodified catalyst composition of claim 25 wherein the refractory oxidesupport is silica having at least 85% of its pore volume in the pores ofabout 250 to about 450 Angstrom diameter.
 27. A modified catalystcomposition of claim 26 wherein the supported catalyst compositioncontains about 0.05 to about 3.0% by weight of chromium, calculated aselemental chromium.
 28. A modified catalyst composition of claim 27wherein the supported catalyst composition contains about 0.10 to about0.25% by weight of chromium, calculated as elemental chromium.
 29. Amodified catalyst composition of claim 28 wherein the supportedcomposition about 0.13 to about 0.18% by weight of chromium, calculatedas elemental chromium.
 30. A modified catalyst composition of claim 29wherein the supported catalyst composition contains about 0.15 to about0.18% by weight of chromium, calculated as elemental chromium.
 31. Amodified catalyst composition of claim 30 wherein the refractory oxidesupport is silica containing 0.18% by weight of CrO₃ and having about88% of its pore volume in the pores of about 250 to about 450 Angstromdiameter.
 32. A modified catalyst composition of claim 31 wherein, priorto the activation of the supported catalyst composition, the refractoryoxide support is dehydrated at a temperature of about 100° to about 500°C. for at least about 14 hours in a nitrogen atmosphere.
 33. A modifiedcatalyst composition of claim 32 wherein the refractory oxide support isdehydrated at a temperature of about 150° to about 300° C. for at leastabout 16 hours.
 34. A modified catalyst composition of claim 33 whereinthe refractory oxide support is dehydrated at a temperature of about200° C. for about 16 hours.
 35. A modified catalyst composition of claim34 wherein, after the dehydration is completed, the titanium compound isdeposited on the dehydrated refractory oxide support.
 36. A modifiedcatalyst composition of claim 35 wherein, after the titanium compounddeposition is completed, the supported catalyst composition is activatedat a temperature of at least about 820° C. for at least about 16 hours.37. A modified catalyst composition of claim 36 wherein the supportedcatalyst composition is activated at a temperature of about 820° toabout 870° C.
 38. A modified catalyst composition of claim 37 whereinthe supported catalyst composition is activated at a temperature ofabout 825° C. for about 16 hours.
 39. A method of preparing a modifiedcatalyst composition comprising combining a supported alpha-olefinpolymerization catalyst composition containing a chromium compound,which is CrO₃ or any compound of chromium calcinable to CrO₃ undercatalyst activation conditions, with a catalyst modifier which is anoxide of an element of Group IIA of the Periodic Chart of the Elementscontaining at least about 0.5% by weight of water.
 40. A method of claim39 wherein the supported catalyst composition additionally comprises atitanium compound which is oxidazable to TiO₂ under catalyst activationconditions.
 41. A method of claim 40 wherein the catalyst modifier hasparticle size distribution range compatible with that of the catalystcomposition.
 42. A method of claim 41 wherein the supported catalystcomposition is substantially non-spherical and irregular in shape, andit has a relatively broad particle size distribution range.
 43. A methodof claim 42 wherein particle diameter of the catalyst modifier is about5 to about 250μ.
 44. A method of claim 43 wherein the particle diameterof the catalyst modifier is about 10 to about 200μ.
 45. A method ofclaim 44 wherein the particle diameter of the catalyst modifier is about45 to about 180μ.
 46. A method of claim 45 wherein the catalyst modifiercontains at least about 1% by weight of water.
 47. A method of claim 46wherein the catalyst modifier contains at least about 1.5% by weight ofwater.
 48. A method of claim 47 wherein the catalyst modifier containsat least about 2% by weight of water.
 49. A method of claim 48 whereinthe modified catalyst composition is used in a fluidized bedpolymerization reaction.
 50. A method of claim 49 wherein the supportedcatalyst composition has the particle diameter of about 5 to about 250μ.51. A method of claim 50 wherein the supported catalyst composition hasthe particle diameter of about 10 to about 200μ.
 52. A method of claim51 wherein the supported catalyst composition is combined with about 1to about 15% by weight of the catalyst modifier.
 53. A method of claim52 wherein the supported catalyst composition is combined with about 3to about 9% by weight of the catalyst modifier.
 54. A method of claim 53wherein the catalyst composition is combined with about 5 to about 7%weight of the catalyst modifier.
 55. A method of claim 54 wherein thesupported catalyst composition has the following particle sizedistribution:cumulative volume distribution--10 to 90% of the catalystparticles have diameters of about 28 to about 150μ, as determined in aHIAC-ROYCO PA-720 particle size analyzer.
 56. A method of claim 55wherein the supported catalyst composition has the following particlesize distribution:cumulative volume distribution--10 to 90% of thecatalyst particles have diameters of about 30 to about 125μ, asdetermined in a HIAC-ROYCO PA-720 particle size analyzer.
 57. A methodof claim 56 wherein the supported catalyst composition has the followingparticle size distribution:cumulative volume distribution--10 to 90% ofthe catalyst particles have diameters of about 37 to about 117μ, asdetermined in a HIAC-ROYCO PA-720 particle size analyzer.
 58. A methodof claim 57 wherein the supported catalyst composition has a medianparticle diameter of about 50 to about 100μ, as determined by volumedistribution using a HIAC-ROYCO PA-720 particle size analyzer.
 59. Amethod of claim 58 wherein the catalyst modifier is selected from thegroup consisting of beryllium oxide, magnesium oxide, calcium oxide,strontium oxide, barium oxide, radium oxide or mixtures thereof.
 60. Amethod of claim 59 wherein the catalyst modifier is selected from thegroup consisting of calcium oxide and magnesium oxide.
 61. A method ofclaim 60 wherein the catalyst modifier is magnesium oxide.
 62. A methodof claim 52 wherein the catalyst composition is combined with 9% byweight of the magnesium oxide.
 63. A method of claim 52 wherein thecatalyst composition is combined with 15% by weight of the magnesiumoxide.
 64. A modified catalyst composition of claim 38 wherein thecatalyst modifier is selected from the group consisting of berylliumoxide, magnesium oxide, calcium oxide, strontium oxide, barium oxide,radium oxide or mixtures thereof.
 65. A modified catalyst composition ofclaim 64 wherein the catalyst modifier is selected from the groupconsisting of calcium oxide and magnesium oxide.
 66. A modified catalystcomposition of claim 65 wherein the catalyst modifier is magnesiumoxide.
 67. A modified catalyst composition of claim 14 which is combinedwith 9% by weight of the magnesium oxide.
 68. A modified catalystcomposition of claim 14 which is combined with 15% by weight of themagnesium oxide.
 69. A modified catalyst composition of claim 1 whichproduces alpha-olefin polymers of a substantially higher HLMI that thesame catalyst composition without the modifier.
 70. A method of claim 39which results in the modified catalyst composition which producesalpha-olefin polymers of a substantially higher HLMI than the samecatalyst composition without the modifier.